Inkjet printing machines or printers include at least one printhead that ejects drops of liquid ink onto the surface of media. An inkjet printer employs inks in which pigments or other colorants are suspended in a carrier or are in solution with a solvent. The solvent may be water based, as in aqueous inks, or non-water based, as in non-aqueous inks. The amount of spread of ink drops ejected onto media affects the quality of the ink image formed. The spread of aqueous ink drops can be large enough to affect image quality adversely, particularly on coated media, such as papers having glossy, semi-glossy, or matte surfaces. These coated media are sometimes called offset paper and the interaction of aqueous ink with the surface of offset paper can be problematic. For example, aqueous ink ejected onto offset papers having glossy or semi-glossy surfaces often does not penetrate the media or get absorbed properly. The glossy or semi-glossy surfaces are water resistant because the surfaces have a relatively low porosity or permeability compared to uncoated papers. Consequently, the individual ink drops, which dry primarily by evaporation of water in the ink, slowly spread laterally across the surface of the coating before drying. If the drops are not dry enough and, consequently, are too mobile after they have joined with adjacent drops, a possibility exists of disturbing or moving the ink in these areas so the basic positions of these drops, the uniformity of the thickness of the inks at various positions, or both are affected. These conditions are commonly known as “coalescing” and “puddling” of the ink drops and noticeably impacts print quality. Because coalesced or puddled ink drops exhibit uneven thicknesses and drying characteristics they may also be undesirably transferred to other surfaces with which the media comes into contact, such as other paper in a sheet fed printing process. Moreover, when inks dry too slowly, two different colors of ink ejected adjacent each other tend to bleed into one another producing a defect known as “intercolor bleed.”
Another example of the interaction of aqueous ink with offset papers occurs with offset papers having a matte finish. This type of surface often absorbs ink too deeply into the paper since the matte surfaces are more porous than glossy paper. Thus, media with matte surfaces permit deeper penetration of ink that other media finishes, especially when aqueous ink is used since aqueous ink has high water content. Accordingly, the printed image may lose color richness. Localized differences in the water content of media can also result in undesirable paper cockle.
To address these issues with offset papers, some inkjet printers apply a primer coating to offset papers before ejecting liquid ink onto the media to control ink spreading while reducing ink coalescence and puddle formation. As used in this document, “primer” refers to a material that when dried or partially dried on the surface of paper or media modifies the surface properties, such as the surface tension and the like so when ink is applied to the surface the ink spreads properly and uniformly coats the media or paper surface. Primers can be applied using a variety of methods, such as spraying, roller application, ejector coating, and the like. The intent of applying primer is generally to cause ink drops to spread more than they would without the primer, but not to spread excessively or to non-uniformly. The application of the primer coating, however, is not always effective as aqueous ink drops sometimes spread excessively on the surface of primer coating. Several factors affecting ink drop spreading include the type of primer coating and ink used, the thickness, weight, or amount of primer applied, dryness and temperature conditions of the media, and the amount of time required to dry the ink after it lands on the media. In some inkjet printers, primer coating has been applied to address ink drop spreading on the media. In some of these cases, however, the primer coating does not consistently achieve adequate ink drop spreading. Thus, a system or method that enables ink spread to be tuned for plastic medias, offset papers, and uncoated media treated with primer is desirable.